1. Field of the Invention
The present invention relates generally to fixed cutter or drag-type bits for drilling subterranean formations and, more specifically, to drag bits for drilling hard and/or abrasive rock formations, and especially for drilling such formations interbedded with soft and nonabrasive layers.
2. State of the Art
So-called “impregnated” drag bits are used conventionally for drilling hard and/or abrasive rock formations, such as sandstones. The impregnated drill bits typically employ a cutting face composed of superabrasive cutting particles, such as natural or synthetic diamond grit, dispersed within a matrix of wear-resistant material. As such a bit drills, the matrix and embedded diamond particles wear, worn cutting particles are lost and new cutting particles are exposed. These diamond particles may either be natural or synthetic and may be cast integral with the body of the bit, as in low-pressure infiltration, or may be preformed separately, as in hot isostatic pressure infiltration, and attached to the bit by brazing or furnaced to the bit body during manufacturing thereof by an infiltration process.
Conventional impregnated bits generally exhibit a poor hydraulics design by employing a crow's foot to distribute drilling fluid across the bit face and providing only minimal flow area. Further, conventional impregnated bits do not drill effectively when the bit encounters softer and less abrasive layers of rock, such as shales. When drilling through shale, or other soft formations, with a conventional impregnated drag bit, the cutting structure tends to quickly clog or “ball up” with formation material, making the drill bit ineffective. The softer formations can also plug up fluid courses formed in the drill bit, causing heat buildup and premature wear of the bit. Therefore, when shale-type formations are encountered, a more aggressive bit is desired to achieve a higher rate of penetration (ROP). It follows, therefore, that selection of a bit for use in a particular drilling operation becomes more complicated when it is expected that formations of more than one type will be encountered during the drilling operation.
Moreover, during the drilling of a well bore, the well may be drilled in multiple sections wherein at least one section is drilled followed by the cementing of a tubular metal casing within the borehole. In some instances, several sections of the well bore may include casing of successively smaller sizes, or a liner may be set in addition to the casing. In cementing the casing (such term including a liner) within the borehole, cement is conventionally disposed within an annulus defined between the casing and the borehole wall by flowing the cement downwardly through the casing to the bottom thereof and then displacing the cement through a so-called “float shoe” such that it flows back upwardly through the annulus. Such a process conventionally results in a mass or section of hardened cement proximate the float shoe and formed at the lower extremity of the casing. Thus, in order to drill the well bore to further depths, it becomes necessary to first drill through the float shoe and mass of cement.
Conventionally, the drill bit used to drill out the cement and float shoe does not exhibit the desired design for drilling the subterranean formation which lies there beyond. Thus, those drilling the well bore are often faced with the decision of changing out drill bits after the cement and float shoe have been penetrated or, alternatively, continuing with a drill bit which may not be optimized for drilling the subterranean formation below the casing.
Thus, it would be beneficial to design a drill bit which would perform more aggressively in softer, less abrasive formations while also providing adequate ROP in harder, more abrasive formations without requiring increased weight on bit (WOB) during the drilling process.
Additionally, it would be advantageous to provide a drill bit with “drill out” features which enable the drill bit to drill through a cement shoe and continue drilling the subsequently encountered subterranean formation in an efficient manner.